(1) Field of the Invention
This invention generally relates to antennas and more specifically to quadrifilar antennas.
(2) Description of the Prior Art
Numerous communication networks utilize omnidirectional antenna systems to establish communications between various stations in the network. In some networks one or more stations may be mobile while others may be fixed land-based or satellite stations. Antenna systems that are omnidirectional in a horizontal plane are preferred in such applications because alternative highly directional antenna systems become difficult to apply, particularly at a mobile station that may communicate with both fixed land-based and satellite stations. In such applications it is desirable to provide a horizontally omnidirectional antenna system that is compact yet characterized by a wide bandwidth and a good front-to-back ratio in elevation with either horizontal or vertical polarization.
Some prior art omnidirectional antenna systems use an end fed quadrifilar helix antenna for satellite communication and a co-mounted dipole antenna for land based communications. However, each antenna has a limited bandwidth. Collectively their performance can be dependent upon antenna position relative to a ground plane. The dipole antenna has no front-to-back ratio and thus its performance can be severely degraded by heavy reflections when the antenna is mounted on a ship, particularly over low elevation angles. These co-mounted antennas also have spatial requirements that can limit their use in confined areas aboard ships or similar mobile stations.
The following patents disclose helical antennas that exhibit some, but not all, the previously described desirable characteristics:
U.S. Pat. No. 5,485,170 (1996) to McCarrick discloses a mobile satellite communications system (SMAT) mast antenna with reduced frequency scanning for mobile use in accessing stationary geosynchronous and/or geostable satellites. The antenna includes a multi-turn quadrifilar helix antenna that is fed in phase rotation at its base and is provided with a pitch and/or diameter adjustment for the helix elements, causing beam scanning in the elevation plane while remaining relatively omni-directional in azimuth. The antenna diameter and helical pitch are optimized to reduce the frequency scanning effect, and a technique is disclosed for aiming the antenna to compensate for any remaining frequency scanning effect.
U.S. Pat. No. 5,701,130 (1997) to Thill et al. discloses a self phased antenna element with a dielectric. The antenna element has two pairs of arms in a crossed relationship to transceive a signal at a resonant frequency. A dielectric is disposed adjacent an arm to obtain a self phased relationship in the arms at the resonant frequency. The arms can form crossed loops or twisted crossed loops such as a quadrifilar helix antenna element. A dielectric collar on arms of the same loop causes currents to be equally spaced from one another. The antenna size is reduced and a cross section of the antenna element appears circular without degradation of a gain pattern when the dielectric is used on a certain arm.
In U.S. Pat. No. 5,721,557 (1998) Wheeler et al. disclose a nonsquinting end-fed quadrifilar helix antenna. Each conductor of the antenna is fed with a successively delayed phase representation of the input signal to optimize transmission characteristics. Each of the conductors is separated into a number, Z, of discrete conductor portions by Z−1 capacitive discontinuities. The addition of the capacitive discontinuities results in the formation of an antenna array. The end result of the antenna array is a quadrifilar helix antenna which is nonsquinting, that is, the antenna radiates in a given direction independently of frequency.
There exists a family of quadrifilar helixes that are broadband impedance wise above a certain “cut-in” frequency, and thus are useful for wideband satellite communications including Demand Assigned Multiple Access (DAMA) UHF functions in the range of 240 to 320 MHz and for other satellite communications functions in the range of 320 to 410 MHz). Typically these antennas have (1) a pitch angle of the elements on the helix cylindrical surface from 50 down to roughly 20 degrees, (2) elements that are at least roughly ¾ wavelengths long, and (3) a “cut-in” frequency roughly corresponding to a frequency at which a wavelength is twice the length of one turn of the antenna element. This dependence changes with pitch angle. Above the “cut-in” frequency, the helix has an approximately flat VSWR around 2:1 or less (about the Z0 value of the antenna). Thus the antenna is broadband impedance-wise above the cut-in frequency. The previous three dimensions translate into a helix diameter of 0.1 to 0.2 wavelengths at the cut-in frequency.
For pitch angles of approximately 30° to 50°, such antennas provide good cardioid shaped patterns for satellite communications. Good circular polarization exists down to the horizon since the antenna is greater than 1.5 wavelengths long (2 elements constitute one array of the dual array, quadrifilar antenna) and is at least one turn. At the cut-in frequency, lower angled helixes have sharper patterns. As frequency increases, patterns start to flatten overhead and spread out near the horizon and small nulls start to form overhead. For a given satellite band to be covered, a tradeoff can be chosen on how sharp the pattern is allowed to be at the bottom of the band and how much it can be spread out by the time the top of the band is reached. This tradeoff is made by choosing where the band should start relative to the cut-in frequency and the pitch angle.
For optimum front-to-back ratio performance, the bottom of the band should start at the cut-in frequency. This is because, for a given element thickness, backside radiation increases with frequency (the front-to-back ratio decreases with frequency). This decrease of front-to-back ratio with frequency limits the antenna immunity to multipath nulling effects.
Other factors that influence the front-to-back ratio include the method of feeding the antenna, the physical size of antenna elements, the dielectric loading of the antenna elements and the termination of the antenna elements. Looking first at antenna feeding, the front-to-back ratio improves when an antenna is fed in a “backfire mode” such that the antenna feed point is at the top of a vertically oriented antenna, as opposed to a “forward fire mode” when the feed point is at the bottom of the antenna.
Thinner elements increase the front-to-back ratio. However, as the elements become thinner, the input impedance to the antenna increases and introduces a requirement for impedance matching. Alternatively, lower impedances can be obtained by constructing an antenna with a partial overlap of the antenna elements to increase capacitance. However, a loss of impedance bandwidth starts to occur since the capacitance is a non-radiating capacitance; that is, no radiation can occur from the overlapped areas of the antenna.
Increasing the dielectric loading of the helix elements decreases the front-to-back ratio. Wide flat elements found in many helix antennas have a pronounced loading if one side of each antenna element touches a dielectric, as in the case where the dielectric is a support cylinder for the antenna. If the gap between adjacent elements is small, the field is strongly concentrated in the gap and any dielectric in the gap will load the antenna strongly. Quadrifilar helix antennas can terminate with open or shorted ends remote from the feed point. It has been found that antennas with open ends have a slightly higher front-to-back ratio than do antennas with shorted ends.
My above-identified pending U.S. patent Ser. No. 08/356,803 discloses an antenna having four constant-width antenna elements wrapped about the periphery of a cylindrical support. This construction provides a broadband antenna with a bandwidth of 240 to at least 400 MHz and with an input impedance in a normal range, e.g., 100 ohms. This antenna also exhibits a good front-to-back ratio in both open-ended and shorted configurations. In this antenna, each antenna element has a width corresponding to about 95% of the available width for that element. However, it was found that this antenna could require a tradeoff between the pattern shapes in the transmit and receive bands. It became necessary to allow patterns at lower receive frequencies to become sharper overhead than desired. At higher transmit frequencies, it became necessary to accept overhead patterns that were flatter overhead than desired. At even higher frequencies, nulls were observed in the patterns because the element lengths were becoming long enough electrically for multilobing to begin.